Crystallization is an important purification and separation technique in a variety of commercial processes, as for example biotechnology, mineral processing, waste treatment, energy storage, new materials and electronic chemicals. Crystallization can occur in solution, from vapor or from melt. Most processes in the chemical industries use crystallization from solution, and the starting point for it is the creation of a saturated solution. However, with the currently known methods the formation of a saturated solution is often a time-consuming process. Usually, it takes days until the equilibrium between the compound's soluble and insoluble forms has been reached. Hence, instead of a saturated solution, a supersaturated solution is used in most instances as the starting point for the crystallization. However, in such cases it is important to know the level of supersaturation, since the supersaturation appropriate for crystallization varies from compound to compound and depends on the compound's ability for crystallization. In general, with decreasing level of supersaturation the crystal growth becomes slower and the crystal quality improves.
Wang has tried to obtain a very high supersaturation of nicotinic acid by adding hydrochloric acid to a sodium nicotinate aqueous solution (Wang, F and Berglund, K A. Monitoring pH swing crystallization of nicotinic acid by the use of attenuated total reflection Fourier transform infrared spectrometry. Industrial & Engineering Chemistry Research 39(6), 2000, 2101-2104). The point of highest supersaturation was then used as the starting point for the nicotinic acid crystallization.
Another example is controlled batch crystallization by pH variation, developed by Zhu (Zhu, J and Garside, J. Controlled batch crystallization by pH variation. Jubilee Research Event, a Two-Day Symposium, Nottingham, UK, Apr. 8-9, 1997, 1, 449-452). According to Zhu, a short pulse of supersaturation initialized crystallization, and during the whole crystallization process, pH change was used in order to maintain a constant level of supersaturation. Furthermore, Zhu tried to raise the level of supersaturation as high as possible in order to shorten operation time.
However, supersaturation is considered to be a risk for the formation of amorphous material and for crystal defect occurrence. Therefore, with the presently known methods for crystallization relying on supersaturated solutions, one has to establish an appropriate level of supersaturation in each case, trying to find a compromise between the rate of crystallization and the quality of crystals formed.